Can you be more specific? Which repos exactly and which pull requests? If you want you can forward one of the emails to email@example.com
Note that even if you are not a writer and you're not watching a repo, somebody can still explicitly add you to a pull request as a reviewer and then you would still get an email. Can you check if that's what happened? You would be listed as a reviewer on the PR overview page.
Actually, is it possible that these pull requests pre-date our recent pull request reviewer changes? The behavior we have now (notifications being sent only if you are a writer and watching the repo) don't retroactively apply to existing pull requests.
In that case you can simply unwatch the PRs in question and that should be it.
Should I really have to actively unsubscribe to each pull request just because I'm following the repo? Maybe I'm misinterpreting "Following" as the equivalent of Github's "Starred"? I'd just like to star the repos like I do with Github so that I can find them easily, not get swarmed by email notifications for every pull request that is created.
The bug that was causing this has been fixed and deployed. As Erik mentions above, this fix will not remove you from watching any existing pull requests (you'll have to do that manually), but it will fix it for any future pull requests.